1. Field of the Invention
This invention relates to digital wireless systems, and most particularly, to wireless communications such as those using CDMA modulation.
2. Description of the Related Art
In certain digital wireless systems, “fast feedback” information should be grouped onto one channel for transmission. This is typically physical layer or very low layer message processing related information which should be transmitted and received quickly, and hence does not go through the normal higher protocol layer flow. The classic example is closed loop power control implemented in CDMA systems to overcome the “near-far” problem. The discussion is relevant for both forward link (base station “BS” transmitting to mobile station “mobile”) and reverse link (vice-versa) data transmission.
In newer proposals for next generation systems various new fast feedback channels have been discussed. In the following, these are the discussed from the mobile perspective (reverse link). These include:    transmit sector indication—(preferably frame synchronous) implements a “fast handoff” by allowing the mobile to decide which of several active basestations will transmit in the next frame. This provides a priori “selection diversity” gain because the mobile can select the basestation that provides the largest signal power. This should be fast to select the best possible basestation based on changing channel conditions.    power control indication—requests increase or decrease of transmitter power in order to provide just enough power to the mobile to allow successful demodulation while minimizing interference to other mobiles. This should be fast to track out the variations of the changing channel. This is typically a simple up/down indication.    fast frame acknowledgments—(preferably frame synchronous) allows the BS to immediately retransmit frames of data received by the mobile in error. The immediate transmission allows the mobile to combine the received symbols from the previous (unsuccessful) transmission with the newly received symbols to improve the probability of successful reception. In order to keep the mobile's memory storage requirements to a minimum this should be done quickly, requiring fast acknowledgments. (This scheme is termed “hybrid-ARQ”).    SNR (Signal-to-Noise Ratio) indications—(preferably frame synchronous) signal to noise indications can be transmitted to the BS to allow it to decide how and what to transmit in the next frame. Specifically, current proposals use adaptive modulation and coding, where the transmit data and coding rate is adjusted according the signal strength received by the mobile. These should be fast to allow the BS to appropriately and quickly select the modulation for the next frame of data. Alternatively, the mobile may directly request a specific data rate.    rate information—(preferably frame synchronous) the mobile can indicate to the BS the data rate of the mobile's transmission, in order to facilitate the BS's decoding of the data. Preferably, this would be transmitted synchronously with each data frame.    adaptive antenna array information—the BS may use transmit adaptive antenna arrays, which typically require some feedback from the mobile in order for the BS to adjust its transmit weights. This should be transmitted fast enough to allow adequate weight adaptation rates.
All of these things should be transmitted and received quickly based on the changing conditions of the environment, and many of them are frame synchronized. These requirements mean that they cannot be treated as “signaling” which passes through the higher protocol layers of the receiver and transmitter. Rather, these can all be considered to be physical layer related data, and as such it is reasonable to consider special physical channels for transmission. In the past, when power control was the only such information, the power control bits where simply “punctured” onto the data transmission (that is, transmit “Tx” symbols of the data were erased and replaced with power control bits which were transmitted in their stead). With the increasing amount of information and the increasing required reliability of the information, new independent physical channels have been considered. This has not, however, been considered from a single, unified, flexible framework.
The present proposals have all included ad hoc physical channels (distinguished by orthogonal codes in CDMA) for these feedback terms. This increases the number of channels, which causes increased transmitter and receiver complexity (for DS-CDMA each physical channel will require one despread-accumulator) and increases the peak-to-average ratio of the transmitted waveform (which increases power consumption in the radio).
It would be desirable to improve upon these proposals.